Review Question - QID 108911

A 68-year-old woman presents to her primary care provider for dizziness. She often feels lightheaded and like she may “pass out” but never actually loses consciousness. She denies dyspnea on exertion or orthopnea. The patient had a myocardial infarction 6 months ago, and she now has multiple drug-eluting stents. Her ECG upon discharge from that hospitalization was normal. The patient has been taking all of her medications as instructed. Her medical history is otherwise significant for rheumatoid arthritis and migraines. Her home medications include methotrexate, atorvastatin, clopidogrel, and aspirin. Her temperature is 98.6°F (37°C), blood pressure is 118/75 mmHg, pulse is 92/min, and respirations are 12/min. On physical exam, she has an irregular heartbeat with normal cardiac sounds. Her abdomen is non-distended and she has mild lower extremity edema. Her current ECG can be seen in Figure A. Which component of the cardiac conduction system is most likely affected in this patient?